The long-term objective of this project is to identify the determinants of catheter ablation failure in AV nodal reentrant tachycardia (AVNNRT) and accessory pathways (AP). Recently, the NASPE Catheter Ablation Registry showed that despite all the advances in mapping and ablation technology, the success rate of ablation has not improved between 1993 and 1998, suggesting that new strategies for selecting the ablation target are needed. The Specific Aim 1 of this project is to elucidate the causes of failed AP ablations. We hypothesize that the main cause of failure in AP ablation is inaccurate localization of the AP when the AP has an oblique course. We will test this hypothesis by reversing the activation wavefront using two different pacing sites to help identify an isolated AP potential, which will be targeted for ablation. The Specific Aim 2 of this project is to elucidate the causes of failed AVNRT ablations. We postulate that the main cause of failed AVNRT ablation is that the reentrant circuit in AVNRT is not well understood. Our working hypothesis is that identification of the reentrant circuit and atrial connections in AVNRT will facilitate appropriate target selection and improve success in AVNRT ablation. We will systemically examine the reentrant circuit by (1) map the earliest retrograde atrial activation to help identify the retrograde limb of the circuit in each variant of AVNRT. (2) establish the presence or absence of a lower common pathway in each variant of AVNRT to help localize the circuit. (3) deliver late atrial extrastimuli at different sites to identify the antegrade limb of the circuit. (4) ablate the reentrant circuit based on the mapping result to further confirm the location of the circuit. My past research has focused on basic electrophysiology (ion channel related research). My immediate career goal is to utilize this project to successfully change my research direction from basic to clinical electrophysiology. My long-term goal is to be a "linker" between basic and clinical electrophysiology and continue exploring the mechanism of cardiac arrhythmia.